Ultrashort pulse laser systems, that is to say laser arrangements which are capable of generating laser pulses having a characteristic pulse duration in the femto- or picosecond range, have been known for a long time in various embodiments from the prior art. Besides complex arrangements having long resonators, microchip lasers are also used in this case.
So-called Q-switched microchip lasers, which emit a pulse length of a few tens of picoseconds in the near infrared, are already known in their basic structure from Spühler G. J. et al. “Experimentally confirmed design guidelines for passively Q switched microchip lasers using semiconductor saturable absorbers”, JOSA B, Vol. 16, No. 3, March 1999. These lasers have the advantage of particular compactness compared with mode-coupled lasers, since the laser resonator itself occupies a volume of only a few cubic millimeters or even less, while even very compact mode-coupled resonators have an edge length which is some centimeters long owing to the required resonator length. A further advantage is that laser pulses can be generated with a lower pulse repetition rate but a higher pulse energy than in the case of commercially available low-power mode-coupled oscillators.
Braun B. et al. “56-ps passively Q-switched diode-pumped microchip laser”, Optics Letters, Vol. 22, No. 6, March 1997, for example, publishes an arrangement which consists of a 200 μm thin laser medium consisting of Nd:vanadate, which is bounded on one side by a laser mirror and on the other side by a saturable absorber mirror, or SESAM (Semiconductor Saturable Absorber Mirror). The published pulse energy is a few tens of nJ. The laser medium is in this case made as a single piece and then positioned between the two end elements, output coupler and SESAM, without bonding being carried out.
It is known from Zayhowski J. J. and Wilson A. L. “Short-pulsed Nd:YAG/Cr4+:YAG passively Q-switched microchip lasers”, OSA/CLEO 2003, that pulse lengths of around 150 ps can be achieved with a sandwich arrangement of Nd:YAG and passive Cr4+:YAG Q-switching. With a high degree of compactness, this arrangement achieves pulse energies extending into the μJ range, but with pulse lengths which are in excess of 100 ps since the passive Q-switching, i.e. the material Cr4−:YAG, necessitate a certain length so that optimization toward shorter pulse lengths is not possible. These lasers are therefore not suitable for applications in which it is necessary to provide ultrashort pulses.
Nodop D. et al. “High-pulse-energy passively Q-switched-quasi-monolithic microchip lasers operating in the sub-100-ps pulse regime”, Optics Letters, Vol. 32, No. 15, August 2007, likewise proposes an arrangement based on SESAM technology, in which a 200 μm thick laser crystal is applied by a spin-on-glass adhesive bonding technique onto the highly thermally conductive SESAM component. A dichroic output coupler, which transmits the pump light and partially couples out the laser light, is then in turn applied on the crystal by coating. This arrangement is not suitable for crystals configured even more thinly in order to achieve even shorter pulses.
Therefore, although picosecond microchip lasers are known from the prior art, they can however only achieve a minimum pulse length of a few tens of picoseconds, the minimum pulse length published being 37 ps. Even such short pulse lengths, however, are still too long for some applications.
In order to be able to generate shorter pulse lengths, the laser medium would have to be configured even more thinly, but this is problematic in terms of manufacturing technology since the production and handling of laser media as components having a thickness of 100 μm or less entails difficulties.
The problem furthermore arises of the different thermal expansion coefficients, which are necessarily encountered in such a miniature resonator owing to the use of different materials and which can lead to thermally induced fracture in the material or disbonding at a boundary layer. Furthermore, the achievable thermal lens, which is necessary for suitable mode formation, is no longer sufficient because of the longitudinally increasing thermal profile and dissipation. Another problem is the fact that the energy densities in a SESAM become too great and optical destruction can result, so that corresponding long-term operation is therefore not possible in solutions of the prior art. Furthermore, an increasingly thin laser medium entails the problem that the gain also decreases. Lastly, there is a further problem because, in the case of very thin laser media, the oscillation antinodes of the pump radiation—when it is reflected back on itself at a surface or interface opposite an entry face—deviates owing to the different wavelengths from the oscillation antinodes of the laser radiation, so that an optimal overlap cannot be ensured or the overlapping even diverges.